invalid read within valid memory region

Question

I'm working on a simple OCR program with Ptreads, but for some weird reason the program crashes on a simple assignment.

The line causing the crash is a simple assignment: int **y=(mask->imdata); all pointers are okay (checked when malloc'ed and when passed to this function and they are the same)

Valgrind gives the following output:

==28948== Thread 2:
==28948== Invalid read of size 4
==28948==    at 0x804913A: PearsonCorrelation (ocr_pool.c:239)
==28948==    by 0x8049342: PearsonCorrelator (ocr_pool.c:275)
==28948==    by 0x80497BF: characterSegmentation (ocr_pool.c:407)
==28948==    by 0x8049A33: lineSegmentation (ocr_pool.c:489)
==28948==    by 0x8049D64: doPage (ocr_pool.c:575)
==28948==    by 0x4076954: start_thread (pthread_create.c:300)
==28948==    by 0x41565ED: clone (clone.S:130)
==28948==  Address 0x49d18f8 is 8 bytes inside a block of size 12 free'd
==28948==    at 0x4023B6A: free (vg_replace_malloc.c:366)
==28948==    by 0x80489ED: freeImage (ocr_pool.c:77)
==28948==    by 0x8049F33: main (ocr_pool.c:610)

As you see, the reported invalid read is of size 4, with an offset of 8 in a region of 12 bytes, how can this be the cause of a segfault?

Edit: Without Pthreads (executing the same function as pthreads_create() calls) it works without any problems. Even when running one thread, it still gives a segfault.

Answer 1

I believe you are probably freeing the memory you're reading from before reading from it, as I would expect that to produce an error from Valgrind like the one above (though don't directly see how that in itself would result in a crash.)

The segmentation fault could be coming from somewhere else in your program if you're writing to memory you've previously released using free.